It is planned to investigate the mechanism of repression of arginine biosynthetic enzymes in Escherichia coli, with emphasis on the translational component of repression. Primarily, two types of in vivo approaches, based on the use of purified arg messages, will be employed: studies of the formation of translational initiation complexes, as a function of regulatory condition, and an exploration of translational repression with the aid of an in vitro protein-synthesizing system. The arg messages will be prepared by in vitro transcription of arg-gene-containing DNA segments derived from transducing-phage DNA's containing argECBH.